Dissolution of Sexual Signal Complexes in a Hybrid Zone between the Swordtails Xiphophorus birchmanni and Xiphophorus malinche (Poeciliidae)

The evolution of sexual signaling systems is influenced by natural and sexual selection acting on complex interactions among traits. Natural hybrid zones are excellent systems for assessing fitness effects on sexual phenotypes. Most documented hybrid zones, however, show little variation in sexual signals. A hybrid zone between the swordtails Xiphophorus birchmanni and Xiphophorus malinche is characterized by numerous recombinants for male sexual traits. Analyses of geographic variation in morphological and isozyme traits in the Río Calnali, Hidalgo, Mexico, reveal an upstream-to-downstream gradient from X. malinche- to X. birchmanni-type traits. A second hybrid zone, likely isolated from the R. Calnali, occurs in the nearby Arroyo Pochutla. Although the presumed female preference for swords predicts the introgression of swords from X. malinche-like populations into hybrid populations, the opposite pattern was observed. Swords are reduced in populations otherwise characterized by X. malinche traits. Sexually dimorphic traits were poorly correlated within individuals, indicating that sexual selection does not act against recombinant phenotypes. Hybrid males also exhibit trait values outside the range of parental variation. These patterns are consistent with predictions that females are permissive, preferring generally conspicuous males without attending to specific features.

Fig. 1.

Representative males from parental and hybrid populations. (A) Xiphophorus birchmanni collected at Huiznopal, Río Conzintla. Note nuchal hump. (B) Xiphophorus malinche from Río Claro stock. (C) hybrid from Calnali-low. Spotted caudal macromelanophore pattern, diagnostic of X. birchmanni, shows evidence of melanosis. The sword is diagnostic of X. malinche, whereas the regular vertical bars are diagnostic of X. birchmanni. (D) Hybrid collected at Calnali-mid. Note irregular bars, diagnostic of X. malinche, and deep body and reduced sword, diagnostic of X. birchmanni

Fig. 2.

Study sites and male morphometric trait distributions. Boxes and whiskers encompass 75% and 90% of trait variation respectively; horizontal line within box indicates median. Outside values and far outside values are indicated by asterisks and open circles, respectively. Lowercase letters indicate significant differences in ANOVA for standard length and in ANCOVA for depth, dorsal fin width, and sword extension length (see Materials and Methods). M_C, Xiphophorus malinche, near Chicayotla; M, X. malinche, Río Claro; H_H, hybrid zone, Calnali-high; H_M, hybrid zone, Calnali-mid; H_L, hybrid zone, Calnali-low; H_T, hybrid zone, Teocapán; P, hybrid zone, A. Pochutla; B, Xiphophorus birchmanni

Fig. 3.

Principal components plot of study populations. Shaded polygons indicate parental species (M_C, Xiphophorus malinche, Chicayotla; M, X. malinche, Río Claro; B, Xiphophorus birchmanni, Río Conzintla). Hybrids: ▪, Calnali-high; •, Calnali-mid; ♦, Calnali-low; ▴, Teocapán. Unfilled symbols indicate regular bars, filled symbols irregular bars, question marks bar status undetermined. Parental species were fixed with respect to bar morphology

Fig. 4.

Sword index and dorsal width ratio in study populations. Shaded polygons indicate parental species (M_C, Xiphophorus malinche, Chicayotla; M, X. malinche, Río Claro; B, Xiphophorus birchmanni). Hybrids: ▪, Calnali-high; •, Calnali-mid; ♦, Calnali-low; ▴, Teocapán

Received: 26 Feb 2002

Accepted: 06 Oct 2002